TAYLORSVILLE — In December 2012, when crews finish reconstructing I-15 in Utah County, they will leave behind pavement that is so durable it is supposed to last for 40 years, according to the Utah Department of Transportation project director.

The road will be nearly 3 feet deep: 12 inches of coarse gravel, topped by 6 inches of open graded gravel that water can seep through, topped by 3 inches of an asphalt-concrete base, topped by 12 to 12.5 inches of Portland Cement concrete that features high-quality gravel and strong binders, project director Dal Hawks said.

Hawks was at UDOT headquarters Thursday to explain to the Utah Transportation Commission his $1.73 billion project called I-15 Core, which will add two lanes in each direction of I-15 for the 20 miles spanning from Lehi's Main Street to Spanish Fork. Ten interchanges will also be reconstructed, and ground-breaking will be in March or April.

The new pavement will need to be routinely maintained, but it won't have to be replaced for decades, Hawks said.

Commissioner Bevan Wilson said that he and his wife took a road trip to some Eastern states recently and were annoyed by the sound of riding over concrete.

"All we heard was thump, thump, thump, thump," he said. "Have we got technology where we don't have seams?"

The solution is steel, Hawks said. In addition to asphalt, concrete and gravel binders, the road will have large pieces of steel under the joints to prevent the road from faulting and separating vertically.

Hawks also discussed major bridge and interchange work along the I-15 corridor, including:

Spanish Fork interchange: Currently, Spanish Fork has separate exits for Main Street and U.S. 6, and exiting and merging traffic make for harried driving conditions. Reconstruction will consolidate to one exit in Spanish Fork, and once drivers are off the freeway, they take separate routes to get to Main or U.S. 6.

Provo Center Street: The bridges are old and curvy. I-15 Core will bring a "rotary" or oval-shaped interchange to the area.

The S curve between Orem University Parkway and Provo Center streets: The freeway in the area consists of bridges under which trains roll. The bridges are old, and I-15 Core will straighten the freeway in the area, making it safer.

University Parkway, Orem: It's a relatively new interchange. With reconstruction, the interchange will gain more traffic lanes.

500 East American Fork and Main Street American Fork: Both interchanges will be rebuilt as "diverging diamonds," a new, European-inspired engineering concept in which traffic lanes crisscross. Traffic will flow easier. The Main Street interchange is half finished. The 500 East bridge was announced Wednesday.

Orem Center, Orem 800 North and Lehi Main: The interchanges will be redesigned as single-point urban interchanges. Referred to by the acronym SPUI in the construction industry, traffic will flow with only one stoplight, similar to 10600 South and University Parkway, except with I-15 Core, traffic will flow under the freeway.

UDOT originally performed a study for I-15 reconstruction from Point of the Mountain to Payson, UDOT Executive Director John Njord said. The Legislature last session gave $1.73 billion to UDOT, which Njord believed would only be enough to reconstruct from American Fork to Provo. But the contractor that UDOT selected, a consortium of construction companies called Provo River Constructors, was able to stretch the dollars on more portions of I-15, he said.

Commuters can visit and sign up for e-mail updates at the I-15 Core's Web site, www.i15core.utah.gov/.

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